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We recently moved into a new home that currently has a Super Hot MG-100 Hydronic Boiler (installed in 1986, when the house was built). Clearly, the boiler is way past its life expectancy and in need of replacement. We had a couple of boiler companies come give us a quote and one of them suggested putting in a Bosch 151 (ZWD-42) Combi Boiler because it's much more efficient and because he noticed that our water heater is also near its life expectancy. He said we'd kill 2 birds with 1 stone and also take up less space.

I did some research on the internet and can't get a clear answer if this unit, or a combi boiler in general, will be suitable for our house. Some sites said it depends on how many gallons per minute of hot water are used throughout the house since this unit can only output 4.0 GPM. Others said it's based on the number of bathrooms in the house and one site even said mentioned calculating the total length of all the baseboard heaters.

I don't know how many GPM of hot water are used throughout the house or how to even figure that out easily, but I did go and measure all the baseboard heaters.

The house has 3 levels and is located in Vancouver, BC, where it can get pretty chilly during the winters. The boiler is located in the basement and supplies hydronic baseboard heating to the basement and main level only. The upper level is heated by electric baseboard heaters. The basement has a total of 38 linear feet of baseboard heaters and the main level has 61 linear feet.

As for our hot water demands, the basement is a rental suite that has a kitchen sink, washing machine and a single bathroom with a stand-up shower. It is currently occupied by 2 people. The upper and main floors are occupied by my wife and I (and hopefully a child in the near future). On the main floor, there is a kitchen sink, laundry sink, washing machine and a half-bathroom (no shower). The upper floor has 2 bathrooms, one with a tub and the other with a stand-up shower.

How would I figure out if we can suffice with a combi boiler? Would this unit work in our household?

Any help or advice is greatly appreciated.
 
The short answer is no it would not be suitable as it will be trying to provide adequate hot water simultaneously to two residences with at least three bathrooms and a number of other hot water outlets. Heating provision isn't normally an issue with combi boilers but hot water is, the quoting boiler companies should really be able to explain that to you if not find and get another quote from a company that can show you the heat loss calculations. Having said that it is possible to use a combi boiler in conjunction with a pressurised hot water storage vessel that should suffice, again find an engineer that understands that concept as I'm not sure you have those types of vessels in Canada.
 
You'll need a hot water store but you can still utilise a condensing boiler.

I am not recommending them as I have no experience of their products but Navien appear to operate in Canada, see the video on this page as they replace the heating boiler while also making provision for when the hot water furnace eventually needs to be replaced.

 
Do you want your heating (CH) and shower/bath (HW) at the same time ?

A Combi will prioritise HW over the Central Heating, You/Wife M/F and Child ? F maybe, other 2 Occupants ? The point is at 7am when its shower time and mainly females spend more time in the bathroom the hot water isnt going to be going around the two accomodation's, Cental Heating pipes, and its going to get cold.

Best bet is to get a unvented system that can push water out to both systems at the same time.
 

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